Liquid level control



May 23, 1950 A.L E. sToRMEN-r 2,508,434

LIQUID LEVEL CONTROL JE. STORMENT BY El L @q A ATTORN EYS May 23, 1950 Filed Jan. 5l, 1945 J. E. s'roRMENT 2,508,434

LIQUID LEVEL CONTROL 2 sheets-sheet 2 INVENTOR J. E. STORMENT ATroRNEYs Patented May 23, 1950 LIQUID LEVEL CONTROL Joseph E. Storment, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Application January 31, 1945, Serial No. 575,507

6 Claims. (Cl. Zim-683.5)

This invention relates to apparatus and methods for the control of the liquid level of an evaporating liquid. In one specifi-c embodiment this invention relates to an automatic means for maintaining a boiling liquid at a constant and desired level in a tower.

A related means and method for accomplishing similar results is shown in my copending application Serial No. 527,121 led March 18, 1944, now Patent No. 2,473,737.

In evaporation processes in which a liquid is continuously fed into a vessel and vapor is re moved therefrom, it is frequently difficult to prevent the liquid-vapor interface from fluctuating considerably whichY often causes some liquid to `go overhead with the vapor or causes an unsteady flow of vapor effluent from the vessel. Liquid when present with the vapor not only decreases the quality of the vapor but also hampers the operation of subsequent equipment; an unsteady vapor ow will result in overburdening and underburdening any subsequent equipment which might be used to process the vapor.

The fluctuating liquid level in a vessel of boiling liquid is the result of increased or decreased evaporation caused by variations in heat input incurred through uncontrollable minor changes in the control instruments and variations in heat losses through the vessel walls from changing atmospheric conditions.

Thus, for the efficient and accurate operation of processes involving the evaporation of liquids both a method and apparatus for maintaining a constant level of the evaporating liquid are much to be desired.

An object of this invention is to provide methods and apparatus for maintaining the level of boiling liquid in a tower constant and also assuring uniform quality and quantity of efliuent vapor leaving the tower.

Another object of this invention is to provide methods of controlling the liquid level in packed and unpacked towers.

Another object of the invention is to provide apparatus for controlling the liquid level in packed and unpacked towers.

Still another object is to provide an automatic method and apparatus for maintaining the level of an evaporating liquid at a desired height in a tower.

Other objects and advantages will be obvious to those skilled in the art from the accompanying specifications, claims and drawings.

In the drawings:

Figure 1 is an elevational view diagrammatically illustrating one application of the present invention to the continuous evaporation of a liquid in a tower with subsequent treatment of the vapor therefrom.

Figure 2 is an elevational view with parts broken away showing diagrammatically a modified method and means of controlling the liquid level in a tower similar to that shown in Figure 1.

In Figure 1 a tower or evaporating chamber 8 is provided with a vapor outlet consisting of line 9, a liquid feed inlet consisting of line 6, and a fluid inlet consisting of line 22.

Liquid to be evaporated is introduced into tower 8 by line 6 at a constant rate by means of a conventional automatic flow regulating valve 3. Liquid is heated to the desired temperature by a steam heating unit 1 which maintains the liquid at a constant temperature by automatic temperature control valve 4 regulating the iiow of steam through heater l.

Any residues or products may be withdrawn :from tower 8 through line II.

A line 22 having an automatic ow control valve 2| and a heater 23 with a conventional automatic temperature control valve 24 are used to introduce a controlling uid to be used to control the liquid level in tower 8, as explained subsequently in the description of the operation. An element 26 is an injection means for the injection of fluid into tower 8 from line 22 and is positioned slightly above the desired liquid level. Element 21 is a gage glass for indicating the liquid level in tower 8.

Element 26 may be constructed in various ways so as to inject vapor upon the surface of an evaporating liquid in tower 8. It may conveniently be a disk with orifices placed on the side adjacent to the liquid surface for the injection of vapor therefrom. Element 26 also may be a spider arrangement with tubes projecting downward through the liquid surface and extending a short distance below the surface. Vapors pass from orifices in the tubes of the spider and only the vapors from that portion of the tube submerged contact the liquid. Obviously many other means of injecting vapors into the surface of a liquid or into a liquid may be employed without departing from the scope of this invention.

A surge tank I9 is provided for the accumulation of condensed vapor from tower 8. An automatic pressure valve l2 is provided for maintaining pressure in evaporator 8 while permitting the flow of vapor through line I4 to a condenser I6 and thence into surge tank I9. The desired pressure in surge tank I9 ismaintained by by- Y invention as'maybe most 3 passing vapor through line l1 through an automatic pressure regulating valve I8 into the surge tank i9.

A fractionator V3| is provided as a means for separating the components of the eiliuent from tower 8 after passing through surge tank i9.

In Figure 2 element ..53 is a tower or evaporating chamber which 'is provided with a vapor out.- let consisting of line 59, a liquid feed inlet consisting of line 56, and level controlling fluid inlets consisting of lines SS and Sil. The essential elementsV of the tower are similar to than@ of Figure 1 and hence it will suiice merely to point outY these elements. rlhe temperatureand rate of flow of liquid feed entering tower 5B are controlled by an automatic rate control valve 53 and aheater 5'! on line 56. control valve 60 is located on the vapor outlet line 59 to maintain pressure in tower 5S. A level .lrllllolling uid enters tower 58 throughline 62 where the fluid is heated to the desired tournerature in. a heater 63, and .then passes through either of lines 66 or t8, as desired. Elements '15 andY 'I6 are injection means for uniformly clis tributing .the level controlling fluid as it enters tower .5.8, lhi-eotion means le ispositioned above thedesired liquid level and inieotins means l5 is positioned below the liquid level in tower 28. :nav comprise a .spider or dish with orifices thereineiinilartothoee deeorihedior Pleure 1 from which iluid is injected into. tower 53. Liquid level vindicator l2 by transmission means i4 .operates flow valve 13. for regulatingv thefow ofthe .controlling fluid .into tower te? Valve le nreierablv operated bv .a float medium ae ln- .di'eated Figure 2 which opone or Closes the valve L3. inthereduired manner to `maintain the Yliquid. level constant in tower 58' but an electrical levelindicator or any other means .may he used to operate valve 'i3 without departing from the scope of. the invention. Element ll.; .is aV gage glass to visually indicate thelevelv of lieu-iid in tower 58. A ,conduit el ie. provided for reinoving residues or nroduets of evaporation from tower 58'. Y y Y Although, as show-nin both Figures 1 and 2, an externalheater is provided for heating the liquid ieed. tothe desired temperature of. evaporation under-the condition present .in the. evanorating chamber.. the liquid in the evanoratingv chamber may be heated to the desired temperature bv direct heating rneans in the chamber itself, such It is as by electricity, steam,y and hot gases. possible the liquid may have suolo. a vapor pressurel under the conditions prevailing fn the evanoratingohamberthat no heating. means may be neeaSearuand infact. insteadof heating the liuuidthaannaratus may be constructed to cool theliquid if necessary...

Y Heaters'i and,.23 oiFigv-ure 1 andlheaters Eland Y 6,3- of Figure. 2 are preierablvV oonstructedsc that eteamenters the top, ofthe. heater through a control; valve. and exhaust steam4 and. condensate is removed at the bottom of. the heater. various designs of heaters maybe' emploredin the present desirable under the conditions present.l Y

vConve.ntional motor control valves are used in Ytheoneration ofthose embodiments of. the invenn tionillustrated in Figures,y l. and 2 to.. oontroltheA litlljofv` dow and, pressured` Obviously, .various other means; to control these, operating condiina-.v he4 employed.; without. departingV scope of --the invention- An automatic pressure Operation. of Figure 1 In the application of the present invention to the evaporation of a liquid feed as diagrammatically illustrated in Figure l, liquid is passed through line 6 at constant flow by means of control valve 3. The liquid is heated in heater 1 to that temperature necessary to evaporate the liquid under conditions prevailing in tower 8. A valve d regulates the amount of steam through heater 'I necessary to heat the liquidl to the desired temperature. Liquid boils in tower, 8 and Y Vthe vapor passes out through line 9. Liquid level in tower 8 is indicated by numeral l0. The desired `pressure is maintained in the tower 8 by means of a, control valve I2.

11.1 Order to rmaintain the liquid level substan tially constant as indicated by level IU, a level controlling fluid from an outside source or from a portion'of the feed through line 5 passes at a Yconstant rate through line 22 to a heater 23. The rate of flow of the level controlling nuid is controlled by a control valve 21. AThe fluid is heated ill heater 'l to a temperature relatively higher than the temperature of the evaporating liquid in tower 8.

A valve 24 controls theamount ofqsteam necessary to heat the fluid to the desired temperature. `The level controlling fluid passes as a vapor Vinto tower 8 and is injected'into. therspace preferably adjacent to the desired liquid level l0 by injection means 25. However, the level controlling fluid may beiniectedas Well into a space some distance Y above or below the Vdesired liquid level without Y liquid and gives up a portion oi its available heat which may be in the form of either sensible or latent heat or both. The vapor which does not contact the liquid and the. vapor which has contacted theliqud but hasnotl condensed passesy out of tower 8 through line 9 along with the vapor of the evaporating liquid therein.

If the liquid level rises, more relatively hot jvapor contacts the evaporating liquid surface as it cornes closer to theinject'on means 26 and thus increases its evaporation. This increased evaporation halts.v the rise of thev liquid level; or causes it to recede. If the liquid levelv recedes, less relatively hot vapor contacts:` the surface of the evaporating liquid as it falls away from the injection means 2t and thus decreases the evaporation of the liquid. The decreased evaporation halts the receding of the liquid level or causes itrto rise.

i The rise or fall of the liquid level is caused by'l a "corresponding greater or'Y lesser quantity of liquid feedbeing evaporated thany is entering tower 8. Tl-ius, by increasing or decreasing the evaporation by the method` described, any substantial change rin the. liquidV level is prevented. whereby the .amount Vof liquid' evaporating is equal toV the amountA of liquid entering. Y f In operationthe flow rate of the liquid feed entering tower 8 through line 6 and heater 1 is preferably slightly 'moreV than the amount that 'would normally be evaporated. in tower 8 without additional heat.supplied from the injection of .the hot vaporsthrough. injection means 26.. In order to prevent the liquid'from rising above the .desired level, additional heat. required to. maintain the evaporation equal. to; the incoming. liquid is supplied by hot. vapor from .line 2'2. through injection meansy 26. Slight .variations in heat .transfer from the Walls oi vthe tower caused by changing atmospheric conditions and uncontrollable variations in the heat supplied to the liquid feed, which would normally cause a, corresponding variation in the rate of evaporation and consequently a rise or fall of the liquid level, will be compensated in the manner described from the available heat of the hot vapor injected adjacent to the liquid level.

The level controlling iiuid injected through line 22 may be the liquid feed or a liquid or vapor from a separate source having a different composition than the liquid feed. Of course, this fluid must be a kind not harmful or detrimental to the process and capable of vaporization at reasonable temperatures. For example, in boiling a water solution, superheated steam may be used advantageously as the level controlling vapor. If a mixture of hydrocarbons is being evaporated, one of the components being vaporized may be recycled as a superheated reflux Vapor through f pipe 33A which may be placed in operation by opening valve 33B to supply the additional heat required to maintain the liquid level.

In case it is desirable to treat the vapor effluent from tower 8, such as the separation of the components of the vapor by fractionation, the vapor eflluent passes from tower 8 through Vlines 9 and I4 to a surge tank I9. Prior to entry into surge tank I9, the vapor effluent is condensed by means of a condensor I5 in line I4. The desired rpressure is maintained on surge tank I9 by by-passing a portion of the vapor from line 9 through line I1 and an automatic pressure control valve i8. Valve I8 is controlled by the pressure in tank I9 so that only sufficient vapor is by-passed through line I1 to surge tank I9 as is necessary to maintain the desired pressure therein. Liquid comprising the condensed vapor eiliuent accumulates in surge tank I9. Although the quantity of vapor effluent may fluctuate because of variations in the amount of level controlling vapor used. in controlling the liquid level, a liquid feed of a constant quantity is introduced into fractionator SI by means of line 21, pump 28, and constant flow valve 29 from tank I9 which absorbs any fluctuation in the quantity of effluent from tower 8. A low-boiling fraction is discharged from fractionator 3I through line 33, and a high-boiling fraction is discharged through line 32. These fractions may be further processed if desired (not shown).

When the level controlling fluid used has a different composition than the vapor of the evaporating liquid the separation of the level controlling iiuid from the vapor product by fractionation may be required, such as illustrated in Figure l. The recovered level controlling fluid can then be recycled through pipe 33A which may be placed in operation by opening valve 33B. Obviously, if further treatment of the vapor effluent from tower 8 is unnecessary surge tank I9 and fractionator 3| of Figure 1 with their accessory equipment may be omitted.

Operation of Figure 2 Since Figure 2 is a modified arrangement of the level control apparatus of Figure l, its operation will be discussed only briefly. Liquid feed passes through line 5S, constant flow valve 53, heater 51, and into evaporating chamber 58. A temperature controlled valve 54 operates heater 51 to maintain the feed at the desired temperature. Liquid evaporates in chamber 58; the vapor product leaves chamber 58 through line 53 and a constant pressure valve 60. Any residue or products may be discharged through a conduit 6I from tower 58.

A level controlling fluid from an outside source or from the liquid feed by line 55 passes through line 62, heater 53 and into chamber 58 by either line 56 or 68. To inject the level controlling fluid above the liquid level in chamber 58, valve 69 is closed and valve 51 is opened; on the other hand, if it is preferred to inject the controlling fluid below the desired liquid level valve 51 is closed and valve 69 is opened. A control valve 64 regulates the amount of steam entering heater S3 to maintain the level controlling fluid at the desired temperature and to vaporize a level controlling liquid if desired. The controlling fluid, either a liquid or a vapor, is injected into chamber 58 through injection means 15 and 15, which may be similar to the type lof injection means previously described.

A float or level indicator 12 operates valve 13 to vary the quantity of level controlling uid entering chamber 58 in such a manner as to maintain the liquid level relatively constant.

It should be noted that the essential difference in maintaining the liquid level in Figures l and 2 is that in the operation of an arrangement similar to Figure 1 the quantity of level controlling iiuid entering tower 3 is substantially constant but the amount of level controlling fluid actually contacting the liquid varies inversely as the distance from the injection means; while in an arrangement similar to Figure 2 the actual contact between level controlling fluid and evaporating liquid is more or less constant but the quantity of level controlling fluid entering chamber 58 is varied with a change in liquid level. Whether the quantity of controlling fluid in Figure 2 is increased or decreased by a change in. the liquid level will depend on several factors, including the temperature of the controlling fluid itself and whether the'controlling uid is injected as a liquid or vapor.

For example, in some cases it may be desirable to inject the level controlling fluid below the desired liquid level through line 58 as a liquid which is relatively colder than the evaporating liquid in the tower. When operating in this manner the liquid feed is introduced into chamber 513` at a slightly lower rate than the liquid would be .normally evaporated under the conditions prevailing in evaporating chamber 58. Float control 12 and valve 13 are so connected by transmission means 14 that when the liquid level rises above the desired height the quantity of relatively cold level 'controlling liquid injected into the evaporating liquid is decreased, and when the liquid level falls below the desired height the quantity of relatively cold level controlling liquid injected is increased. Because of the heat absorbed by the relatively cold liquid which is injected into the evaporating liquid in varying amounts depending on the height of the liquid level, the evaporation is increased when the liquid level rises and the evaporation is decreased when the liquid level falls. If the level controlling uid is injected into `chamber 58 as a relatively cold vapor instead of liquid through line 58, the quantity of vapor is controlled in the same manner as when using a level controlling liquid. The relatively cold vapor absorbs heat from the relatively hot evaporating -liquid and passes out of the evaporating chamber v58 with the vapor product.

Level controlling vapor which is at a relatively higher temperature than the evaporating liquid in chamber 58 may also be used to control the ascenso Y Y Y a liquid level; by the arrangement illustrated in Figure 2. The level controlling vapor may be'infjected intoz chamber` 58.- either through line; 66 and: injectionrrieansA 'i4 above the desired liquid level or" through: line 68E andA means .l5 below the desired liquidl level. Valve T3 by means of oat or level control 'i2T increases the quantity of relatively hot vapori injected when the liquid level rises and decreases the quantity Ior vapoifwhen the' liquid level falls.

The embodiment of the` invention illustrated in Figure 1 doesA not requirel a mechanicalv liquid levelr indicating means and hence would be especially applicable tothe mixed-phase isomerization process;V such as described in patent byR. W.

Henry, Patent No. 2,366,028, where a boiling hy-V a hydrocarbon liquid mixtureI containing normal Vbutano in ay reaction chamber inthe presence of an aluminum halidel catalyst under appropriate cond-itions'oi' temperature, pressure, etc. Isoou'- tane is withdrawn from the reaction chamber with the vaporY eiiluent and subsequently separated by fractionation. A porti-on yof the isobutane may berecycled to the reaction chamber as an superhe'ated reflux vapor to control the liquid level of the evaporating: hydrocarbon mixture in the reaction chamber similar to the manner de scribed' herein.. Y

In general, the available heat supplied by Ythe level controlling fluid'itself is about 2 to 5y per cent ofY the4 total" heat required to evaporate the liquidi at a rate equivalent to the rate of feed entering the evaporating chamber. However, as much as or more of the heat of evaporation may be supplied by the level controlling fluid Y if necessary.

To those'skilled in theart it will appear evident that-other' types' ofapparatus or elementsthereof capable-of'achieving the same result may be substituted for' portions' of the apparatus in this in- Vention andl it will also appear` that certain apparatus may be omitted under some conditions 'of operation such as the' omission of surge tank I3 and fractionatoi" 3f in Figure 1. These, however; constitute onlyv minor changes inoperatien and are natural-ly' within'the scope of they inven Y tion'. Y

Havingv described. preferred forms: oi the. pres# ent invention and having 4Vpointed out the-primciple considerationsto? beobserved in the.Y con- K struction and operation oiequivalent systemsyit stant rate,heat exchange meansv tocontrol the teinperatureof said liquid Vinfsaid liquid inlet line Vat a constant temperature, a spray head in the chamber, intermediatethe endsthereof, a superheated vapor inlet Vline" connected tof saidfspray head' in the chamber,` meansY to control therrate of Y flow oiV superheated' vaporL in said superheated vaporV inletrline'at; a `constant rate, heatl exof said chamber, means` to control the'rate of Yilow. ofiliquid in said liquid inlet line at a; conchange meansztcr control the-temperature otsad superheated vapor ata constant superheated temperature; saidzspray head. being positioned in said chamberatalpoint intermediate the'top and bottomY off` said chamber and.v spaced above the desired level of. liquid; in. said chamber,l and an outlet conduit connected to the top of said chamber for the evaporated liquid.

2. evaporatorcomprising. a chamber, a liquid inlet Iineconnected to. the bottom of' said chamber, means to control the rate of flow oi liquid in said liqudr'inletY line, heat exchange means to control' the temperature of' said liquid Y in said liquid inlet line, a spray head in said chamber; intermediate theends thereof, a superheated'vapor inlet line connected to Vsaid spray head in the chamber, yflow VcontrolV meansY in saidsup'erheate'dj vapoirlinefto: control the rate of flow of superheated'vap'orin said superheated vapor inlet line, heat exchange' meansV tocontrol the temperature of said superheated vapor, said spray head being positioned/in said chamber at a point intermediate? the top Aand bottom of said chamber and spaced vabovethe desired VlevelV of liquid in'said'chamber; andan outlet conduit connected to the toplof said-chamber for the evaporated liquid. Y Y

3. A mixed? phase isomeriZati-on' process for the isomerization'crf normali butane' to isobutane comprising introducing. liquid normalV butano into an evaporating Zone@ containinggpacking and an aluminum halide catalyst deposited onV saidA packl ing, said iquidbutarie'being introduced toy said zone' at a temperature' aud'at a rate selected so that While rapidevaporationfoccurs the liquid level wil graduallyirisein said: zone', Vand arresting the rise of' said? liquid`v level'. hyforming a barrier oi" a superbeatedY vaporV inv saidy Zone at a temperature in eXc'essoi the boiling" point of said' liquid at a point spaced. above'fsaid desired' liquid' level, whereby said liquid' lever 'is controlled at a level where heat' exchange with? ,said superheated vapor provides suicientthermal unitsr to complete the evaporation off the risingl'iquid', isomerizing a portion oi said normalV butane to isobutane in said zone, and removing said isobutane as a vapor.

fl.V An evaporator ccm'pri'sing'a chamber; a liquidi inlet iineconnected toy said champ er, rst controlV meansin said' 'liquid inlet line adapted to maintain a constant ow rate therethrough, first heat exchange means'in said Yliquid iniet'iine, heat inputcontrol means in said' iirst heat eX- change means operatively connected to said liquid inlet line and responsive tol the' temperature oiliquid in said liquid inlet line downstream of. said "first heat exchange means, injection means in saidV chamber' intermediate the' ends of said means, andA anroutlet conduit connected to theY top of said chamber,

51A-n evaporator Ycomprising a chamber, a

' Vliquid'inlet line connected to the bottom ofsaid chamberyrst control meanslin said liquid inlet line adapted to maintain "a,Y constant flew rate therethrough, rstheat exchange means in said liquid inlet line; heatV input controlmeans in. said rst heat exchange means operatively connected to said liquid inlet line and responsive to the temperature of liquid in said liquid inlet line downstream of said iirst heat exchange means, injection means in said chamber intermediate the ends of said chamber, a vapor inlet line connected to said injection means, second ow control means in said vapor inlet line adapted to maintain a constant ow rate therethrough, second heat exchange means in said vapor inlet line, heat input control means in said second heat exchange means operatively connected to said vapor inlet line and responsive to the temperature in said vapor inlet line downstream of said second heat exchange means, and an outlet conduit connected to the top of said chamber.

6. An evaporator comprising a chamber, a liquid inlet line connected to the bottom of said chamber, first heat exchange means in said liquid inlet line, heat input control means in said iirst heat exchange means operatively connected to said liquid inlet line and responsive to the temperature of liquid in said inlet line downstream of said iirst heat exchange means, injection means in said chamber intermediate the ends of said chamber, a superheated vapor inlet line connected to said injection means, second heat exchange means in said vapor inlet line, heat input control means in said second heat exchange means operatively connected to said vapor inlet 10 line and responsive to the temperature of vapor in said vapor inlet line downstream of said second heat exchange means, and an outlet conduit connected to the top of said chamber.

JOSEPH E. STORMENT.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 50,935 Johnston Nov. 14, 1865 1,585,791 Ruths May 25, 1926 1,682,674 Hedlund Aug. 28, 1928 1,770,256 Smekal July 8, 1930 1,781,836 Flatt Nov. 18, 1930 1,802,531 Prichard Apr. 28, 1931 2,104,310 Roelfsema July 4, 1938 2,222,575 Schutte Nov. 19, 1940 2,263,687 Wunsch Nov. 25, 1941 2,322,102 Gschwind June 15, 1943 2,366,028 Henry Dec. 26, 1944 2,375,321 Nysewander et al. May 8, 1945- 2,388,931 Nelson Nov. 13, 1945 2,403,107 McAllister July 2, 1946 FOREIGN PATENTS Number Country Date 554,742 Great Britain July 16, 1943 

3. A MIXED PHASE ISOMERIZATION PROCESS FOR THE ISOMERIZATION OF NORMAL BUTANE TO ISOBUTANE COMPRISING INTRODUCING LIQUID NORMAL BUTANE INTO AN EVAPORATING ZONE CONTAINING PACKING AND AN ALUMINUM HALIDE CATALYST DEPOSITED ON SAID PACKING, SAID LIQUID BUTANE BEING INTRODUCED TO SAID ZONE AT A TEMPERATURE AND AT A RATE SELECTED SO THAT WHILE RAPID EVAPORATION OCCURS TO LIQUID LEVEL WILL GRADUALLY RISE IN SAID ZONE, AND ARRESTING THE RISE OF SAID LIQUID LEVEL BY A FORMING A BARRIER OF A SUPERHEATED VAPOR IN SAID ZONE AT A TEMPERATURE IN EXCESS OF THE BOILING POINT OF SAID LIQUID AT A POINT SPACED ABOVE SAID DESIRED LIQUID LEVEL, WHEREBY SAID LIQUID LEVEL IS CONTROLLED AT A LEVEL WHERE HEAT EXCHANGE WITH SAID SUPERHEATED VAPOR PROVIDES SUFFICIENT THERMAL UNITS TO COMPLETE THE EVAPORATION OF THE RISING LIQUID, ISOMERIZING A PORTION OF SAID NORMAL BUTANE TO ISOBUTANE IN SAID ZONE, AND REMOVING SAID ISOBUTANE AS A VAPOR. 